INFLUENCE OF EXTERNAL STIMULI. GRAVITY 219 



separated from those in which the nature and manner of the construction, 

 and not merely the position of an organ, are conditioned by the external 

 influence. We may here specially mention that those external conditions 

 under which normally the life-capacity of an organ displays itself, by 

 which, in brief, it is 'determined,' may also as stimuli bring about the 

 appearance of the organs. Roots ordinarily live in a moist substratum in 

 absence of light ; absence of light and presence of moisture bring about also 

 formation of roots. Analogous cases will be frequently brought forward in 

 what follows. 



A. INFLUENCE OF GRAVITY. 



Hofmeister 1 in particular ascribed great importance to the influence 

 of gravity in determining the relationships of configuration of plants, and he 

 referred in support of his view to the dorsiventral construction of the lateral 

 shoots of many plants 2 , the configuration of the leaf of plagiotropous 

 shoots of Begonia, and others. There is now no doubt that the influence 

 of gravity as a ' formative stimulus ' has been greatly over-estimated, and 

 I may refer in this connexion to what has been said in the chapter upon 

 relationships of symmetry and also to what I say regarding the influence 

 of light when what is known about the occurrence of anisophylly is told. 

 Gravity is however in a number of cases of importance partly in relation 

 to the disposition, partly in relation to the construction of the organ. 



I. INFLUENCE OF GRAVITY UPON THE DISPOSITION OF ORGANS. 



That gravity has an influence of the kind we shall refer to here has 

 been concluded from what is known of the formation of organs in the 

 embryo of the Pteridophyta, especially of the Filicineae. In the fertilized 

 egg in this group there arise the following parts stem-apex upwards, 

 root on the under side, one or two cotyledons, and a haustorium which 

 is commonly called the ' foot.' It has however been shown that the lie 

 of the embryo in the archegonium is a definite one, and that the dis- 

 position of these parts is entirely due to internal causes, neither gravity 

 nor light have anything to do with it 3 . If prothalli which float upon 

 the surface of water are illuminated from below, the archegonia which 

 normally appear upon the under side are developed upon the upper 

 side, yet, notwithstanding this inversion of position, the embryos which 

 arise within them have the normal relationships to them. Leitgeb was 

 only able to prove a very limited influence of gravity upon the dis- 



1 Ilofmeister, Allgemeine Morphologic. 



2 Sachs (Text-book, Engl. Ed., p. 208) has shown that a direct effect of gravity upon the arrange- 

 ment of the parts of the bud in the dorsiventral shoots of some trees (Cercis, Corylus) is impossible. 



3 See Heinricher, Beeinflusst das Licht die Organanlage am Farnembryo ? in Mitth. a. d. botan. 

 Inst. zu Giaz. Jena, iSSS. 



